In optical imaging of biological tissues, especially the living human eye, it has been shown in recent years that both optical coherence tomography (OCT) and confocal scanning laser imaging systems have particular individual advantages.
Confocal scanning laser imaging has been successfully applied to retinal imaging and is now well accepted by ophthalmologists in imaging the anatomic structures of the retina (see for example, Sharp, P. F. et al. (2004). “The scanning laser ophthalmoscope—a review of its role in bioscience and medicine.” Physics in Medicine and Biology 49(7): 1085-1096). The depth resolution of a confocal scanning laser ophthalmoscope (CSLO) is determined by the depth of focus of the confocal optics, and as a result, it typically has an axial resolution of approximately 300 microns.
In contrast to CSLO, the axial resolution of OCT is determined by the coherence length of the light source used, and thus can provide a much higher axial imaging resolution—on the order of 10 microns. OCT is particularly useful for diagnostics that require high depth resolution tomographic imaging (Fujimoto, J. G. et al. (2000). “Optical coherence tomography: an emerging technology for biomedical imaging and optical biopsy.” Neoplasia 2(1-2): 9-25; Rollins, A. M. et al. (2002). “Emerging clinical applications of optical coherence tomography.” Optics and Photonics News 13(4): 36-41; Fujimoto, J. G. (2003). “Optical coherence tomography for ultrahigh resolution in vivo imaging.” Nature Biotechnology 21(11): 1361-1367).
Although OCT measures optical reflectivity over a volume of interest, attempts have been made to use the data to produce en-face images that look like the retinal images from an ophthalmoscope (Podoleanu, A. G. et al. (1997). “Simultaneous en-face imaging of two layers in the human retina by low-coherence reflectometry.” Optics Letters 22(13): 1039-1041; Podoleanu, A. G. et al. (1998). “En-face coherence imaging using galvanometer scanner modulation.” Optics Letters 23(3): 147-149). In principle, OCT can be used to measure optical reflectivity at a dense set of points covering a volume of interest, and then the OCT data can be reduced to simulate the en-face image that would be seen by a CSLO or other ophthalmoscope. In one approach, the OCT signal is used to simultaneously create, in addition to an OCT image, an anatomic structure image with an appearance similar to that of a CSLO image. This can be done by integrating, or superimposing on top of one another, multiple OCT signals along the depth dimension. The result is an en-face image based on the averaged overall depth reflectivity for each transverse pixel (Hitzenberger, C. K. et al. (2003). “Three-dimensional imaging of the human retina by high-speed optical coherence tomography.” Optics Express 11(21): 2753-2761). Another approach to generating a CSLO-like image uses multiple light sources of different coherence lengths. This allows simultaneous generation of multiple OCT images of different depth resolutions. Still another approach uses low-pass electronic filtering of the OCT signal to extract a CSLO-like image.
However, these en-face images generated from OCT data have a number of drawbacks. In particular, the image quality is typically inferior to traditional confocal images. OCT collection optics generally collect less of the light returning from the sample than CSLO collection optics. As is well known in the art, OCT is an interference-based technique, so only light that is spatially coherent contributes to the signal. The light returning from the sample is also often collected in a single-mode optical fiber. Reflected light that does not couple into that single mode, either because it falls outside the fiber core or because it enters at too steep an angle, is rejected. Often the vast majority of the light reflected from the sample is rejected in this way. In addition, the frame rate of such en-face images is generally constrained by the speed of the OCT scanning optics.
Given these drawbacks of en-face images generated from OCT data and the different advantages of OCT and CSLO, it is desirable to simultaneously generate both OCT and CSLO images. One of the major benefits of combining OCT and CSLO is that they have different depth ranges. Another is that a 2-D image of the sample generated by CSLO can be used to correctly position the OCT system relative to the sample.
Most designs that have been used to generate both OCT and 2-D images of a sample add an additional optical path to an OCT system to support an independent fundus camera, CSLO, line scanning laser ophthalmoscope (LSLO), or similar imaging modality. Nevertheless, attempts have been made to directly use an OCT configuration to generate both an OCT image and a CSLO image. In one approach, the reference light of an OCT interferometer was alternately temporarily blocked to generate a CSLO signal and restored to generate an OCT signal. However, this approach results in sequential, rather than simultaneous, acquisition of the OCT and CSLO images.
Such a technique, which directly uses the OCT configuration to generate both an OCT image and a CSLO image, collects light for both the OCT and the CSLO images through a small pin-hole defined by the core size of a single-mode fiber. The pinhole is on the order of 10 microns. In contrast, the standard pinhole size in a CSLO system is approximately 100 microns. The numerical aperture (NA) is thus much smaller than in a standard CSLO system, resulting in a lower signal-to-noise ratio for a CSLO image generated with an OCT system. Another problem associated with this approach is that the fiber end can strongly reflect the light returning from the sample. The reflected light can be sent to the confocal detector with a strength greater than the light not reflected from the fiber returning from the sample, causing the CSLO sample signal to be overwhelmed by the fiber end reflection.
A second approach that has been used to generate both OCT and CSLO images is to separate the light returned from the sample into two components using a plate beam-splitter. One of the components is used to generate the OCT image, and the other is directed to a separate pinhole and used to generate a CSLO image. Such a design has some advantages over the alternating generation of OCT and CSLO images described above. Because the light used to generate the CSLO image is directed through a separate pinhole, the size of the pinhole can be chosen to optimize the CSLO signal-to-noise ratio. Further, because the light used to generate the CSLO image is not directed toward the single-mode fiber, the CSLO signal is not overwhelmed by reflection from the fiber end. This approach also allows for simultaneous generation of OCT and CSLO images. Finally, this approach allows the same transverse scanner to be used for both the OCT beam and the CSLO beam. Registration between the OCT and confocal images can be achieved with optical alignment of the respective detectors and the free-space beam splitter.
However, this second approach, too, has significant limitations. Because a portion of the returned sample beam is deflected to the CSLO pinhole, the OCT signal strength is reduced. It also does not maximize the signal to the CSLO detector because the cladding of the OCT single mode fiber absorbs a significant amount of the returned sample beam that could have been used for the CSLO image.
In U.S. Pat. No. 7,382,464, a novel dual-waveguiding module is disclosed for efficient collection and separation of OCT and CSLO signals. The two signals are separated by channeling most of the multi-mode guided optical power to a CSLO detector. The non-tapped single-mode guided optical wave is further sent to a pure single-mode fiber of a standard OCT system for OCT image generation. That invention achieves highly efficient optical power usage and hence high signal to noise ratio, together with inherent pixel-to-pixel registration of the OCT and CSLO images, and a cost reduction of the combined OCT/CSLO system. A specialized fiber optic is required for this approach.
Here we present another approach to generating simultaneous high quality OCT and CSLO images without use of this specialized fiber optic.